Isocyanate is the functional group with the formula R–N=C=O.
Organic compounds that contain an isocyanate group are referred to as isocyanates.
An isocyanate that has two isocyanate groups is known as a di-isocyanate.
Di-isocyanates are manufactured for reactions with polyols in the production of polyurethanes,
a class of polymers.
Isocyanates should not be confused with cyanate esters and isocyanides, whose behaviors are
very different.
The cyanate functional group is arranged opposite of the isocyanate group.
Isocyanides have the connectivity R-N≡C, lacking the oxygen of the cyanate groups.
Production Isocyanates are produced by treating amines
with phosgene: RNH2 + COCl2 → RNCO + 2 HCl
These reactions proceed via the intermediacy of a carbamoyl chlorideCl).
Owing to the hazards associated with phosgene, the production of isocyanates requires special
precautions.
Reactivity Reactions with nucleophiles
Isocyanates are electrophiles, and as such they are reactive toward a variety of nucleophiles
including alcohols, amines, and even water.
Upon treatment with an alcohol, an isocyanate forms a urethane linkage:
ROH + R'NCO → ROC(O)N(H)R' If a di-isocyanate is treated with a compound
containing two or more hydroxyl groups, such as a diol or a polyol, polymer chains are
formed, which are known as polyurethanes.
Isocyanates react with water to form carbon dioxide:
RNCO + H2O → RNH2 + CO2 This reaction is exploited in tandem with
the production of polyurethane to give polyurethane foams.
The carbon dioxide functions as a blowing agent.
Isocyanates also react with amines to give ureas:
R2NH + R'NCO → R2NC(O)N(H)R' The addition of an isocyanate to a urea gives
a biuret: R2NC(O)N(H)R' + R"NCO → R2NC(O)NR'C(O)NHR"
Reaction between a di-isocyanate and a compound containing two or more amine groups, produces
long polymer chains known as polyureas.
Cyclization Isocyanates also can react with themselves.
Aliphatic di-isocyanates can form trimers, which are structurally related to cyanuric
acid.
Isocyanates participate in Diels-Alder reactions, functioning as dienophiles.
Rearrangement Reactions Isocyanates are common intermediates in the
synthesis of primary amines via hydrolysis: Hofmann rearrangement, a reaction in which
a primary amide is treated with a strong oxidizer such as sodium hypobromite or lead tetraacetate
to form an isocyanate intermediate.
Schmidt reaction, a reaction where a carboxylic acid is treated with ammonia and hydrazoic
acid yielding an isocyanate.
Curtius rearrangement degradation of an acyl azide to an isocyanate and nitrogen gas.
Lossen rearrangement, the conversion of a hydroxamic acid to an isocyanate via the formation
of an O-acyl, sulfonyl, or phosphoryl intermediate.
Common isocyanates
The global market for diisocyanates in the year 2000 was 4.4 million tonnes, of which
61.3% was methylene diphenyl diisocyanate, 34.1% was toluene diisocyanate, 3.4% was the
total for hexamethylene diisocyanate and isophorone diisocyanate, and 1.2% was the total for various
others.
A monofunctional isocyanate of industrial significance is methyl isocyanate, which is
used in the manufacture of pesticides.
Hazards Methyl isocyanate was the causative agent
in the Bhopal Disaster that killed thousands of people.
Isocyanates are potentially dangerous irritants to the eyes and respiratory tract, despite
their relatively low acute toxicities.
LD50's are typically several hundred milligrams per kilogram.
Polyurethanes have variable curing times, and the presence of free isocyanates in foams
vary accordingly.
All major producers of MDI and TDI are members of the International Isocyanate Institute,
whose aim is the promotion of the safe handling of MDI and TDI.
Isocyanates are associated with increased risk of non-small cell lung cancer in particular
squamous cell.
See also Isothiocyanate
Polyurethane Methylisocyanate
Polyols Fire classes
References
External links NIOSH Safety and Health Topic: Isocyanates,
from the website of the National Institute for Occupational Safety and Health
Health and Safety Executive, website of the UK Health and Safety Executive, useful search
terms on this site — isocyanates, MVR, asthma
[1] International Isocyanate Institute http:www.actsafe.cauploadspdf/Isocyanates.pdf
Isocyanates – Measurement Methodology, Exposure and Effects Isocyanates in Working Life Workshop
Article Health and Safety Executive, Guidance Note
Isocyanates: Toxic Hazards and Precautions The Society of the Plastics Industry – Technical
Bulletin AX119 MDI-Based Polyurethane Foam Systems: Guidelines for Safe Handling
and Disposal An occupational hygiene assessment of the
use and control of isocyanates in the UK by Hilary A Cowie et al. HSE Research Report
RR311/2005.
Prepared by the Institute of Occupational Medicine for the Health and Safety Executive
Air Sampling Isocyanates